The present invention relates to an ink jet head for ejecting ink to fly the ejected ink toward a recording medium, and an ink jet recording apparatus for recording an image corresponding to image data on a recording medium using the ink jet head.
An ink jet recording apparatus serves to eject ink through ejection orifices to record an image corresponding to image data on a recording medium. Examples of known ink jet recording apparatuses include an electrostatic type, thermal type, and piezo type ink jet recording apparatuses which are classified depending on differences of means for controlling ejection of ink.
Hereinafter, the electrostatic ink jet recording apparatus will be described as an example. The electrostatic ink jet recording apparatus is such that ink containing charged color particles is used, and predetermined voltages are respectively applied to ejection portions of an ink jet head in correspondence to image data, whereby ejection of the ink from the ink jet head is controlled by utilizing electrostatic forces to record an image corresponding to the image data on a recording medium. Known as an example of the electrostatic ink jet recording apparatus is an ink jet recording apparatus disclosed in JP 10-138493 A.
FIG. 7 is a schematic view showing a construction of an example of an ink jet head of an electrostatic ink jet recording apparatus disclosed in JP 10-138493 A. In an ink jet head 80 shown in the figure, only one ejection portion of the ink jet head disclosed in JP 10-138493 A is conceptually shown. The ink jet head 80 includes a head substrate 82, an ink guide 84, an insulating substrate 86, a control electrode 88, a counter electrode 90, a D.C. bias voltage source 92, and a pulse voltage source 94.
Here, the ink guide 84 is disposed on the head substrate 82, and a through hole (ejection orifice) 96 is bored through the insulating substrate 86 so as to correspond in position to the ink guide 84. The ink guide 84 extends through the through hole 96, and its projecting tip portion 84a projects upwardly and beyond a surface of the insulating substrate 86 on a side of a recording medium P. In addition, the head substrate 82 is disposed at a predetermined distance from the insulating substrate 86. Thus, a passage 98 of ink Q is defined between the head substrate 82 and the insulating substrate 86.
The control electrode 88 is provided in a ring-like shape on the surface of the insulating substrate 86 on the side of the recording medium P so as to surround the periphery of the through hole 96 of every ejection portion. In addition, the control electrode 88 is connected to the pulse voltage source 94 for generating a pulse voltage in correspondence to image data. The pulse voltage source 94 is grounded through the D.C. bias voltage source 92.
In addition, the counter electrode 90 is disposed in a position facing the tip portion 84a of the ink guide 84 and is grounded. The recording medium P is disposed on a surface of the counter electrode 90 on a side of the ink guide 84. That is to say, the counter electrode 90 functions as a platen for supporting the recording medium P.
During the recording, the ink Q containing color particles which are charged at the same polarity as that of a voltage applied to the control electrode 88 is made to circulate through the ink passage 98 from the right-hand side to the left-hand side in the figure by a circulation mechanism for ink (not shown). In addition, a high voltage of 1.5 kV for example is continuously applied to the control electrode 88 by the D.C. bias voltage source 92. At this time, part of the ink Q within the ink passage 98 passes through the through hole 96 of the insulating substrate 86 by a capillary phenomenon or the like to be concentrated at the tip portion 84a of the ink guide 84.
If a pulse voltage of 0 V for example is applied from the pulse voltage source 94 to the control electrode 88 biased at 1.5 kV by the bias voltage source 92, then a voltage of 1.5 kV obtained by superposing both the voltages on each other is applied to the control electrode 88. In this state, an electric field strength in the vicinity of the tip portion 84a of the ink guide 84 is relatively low, and hence the ink Q containing the charged color particles which are concentrated at the tip portion 84a of the ink guide 84 is not flied out from the tip portion 84a of the ink guide 84.
On the other hand, if a pulse voltage of 500 V for example is applied from the pulse voltage source 94 to the control electrode 88 biased at 1.5 kV, then a voltage of 2 kV obtained by superposing both the voltages on each other is applied to the control electrode 88. As a result, the ink Q containing the charged color particles which are concentrated at the tip portion 84a of the ink guide 84 is flied out in a form of an ink droplet R from the tip portion 84a of the ink guide 84 by the electrostatic force, and be electrostatically drawn by the grounded counter electrode 90 to be stuck onto the recording medium P to form thereon a dot of the charged color particles.
In such a manner, a recording is carried out with the dots of the charged color particles while the ink jet head 80 and the recording medium P supported on the counter electrode 90 are relatively moved to thereby record an image corresponding to the image data on the recording medium P.
Now, in the electrostatic ink jet head, when a plurality of ejection portions are disposed in a matrix to construct a multi-channel head, it becomes difficult to connect signal wirings to the control electrodes for the respective ejection portions. For this reason, in the case where there is a large number of channels, it is conceivable that the insulating substrate is made in the form of a multilayer wiring structure in order to connect signal wirings to control electrodes. Consequently, in the future, the insulating substrate has a tendency to become gradually thicker along with an increase in the number of channels.
However, since a length of the through hole (ejection orifice) becomes large relatively to an orifice diameter thereof if the insulating substrate is thickened, a resistance between the ink and an inner wall of the through hole becomes large and hence the ink becomes hard to be ejected. In addition, if the insulating substrate is thickened as compared with a velocity of an ink flow, then the ink stays in the through hole to degrade the property of supply of the ink to the tip portion of the ink guide. As a result, there is encountered a problem that responsivity to an ejection frequency becomes poor, and the dot diameter gradually becomes smaller as the drawing speed is further increased.
Note that while not limited to the electrostatic ink jet recording apparatus, when the insulating substrate is thickened, i.e., the length of the through hole becomes large, the same problem occurs in the ink jet recording apparatuses using the various type ink jet heads.